The present invention relates to an acoustically resistive layer for an acoustical attenuation panel, more particularly adapted for panels in the form of conduits for aircraft jet engines.
Acoustic attenuation panels are more particularly adapted to be mounted in the walls of nacelles of aircraft jet engines, in the frames of the reactors or in conduits which must be soundproofed.
In practice, this type of panel includes a cellular core, such as a honeycomb structure covered, on the incident sound wave side, with an acoustically resistive layer, and, on the opposite side, with a rear reflector. As a modification, this type of panel can comprise a superposition of honeycomb structures separated by acoustically resistive layers.
An acoustically resistive layer is a porous structure with a dissipating role, which is to say partially transforming the acoustic energy of the sound wave passing through it, into heat.
This acoustically resistive layer is characterized particularly by a quantity of open surface which varies essentially as a function, on the one hand, of the engine and, more particularly, of its pressure field, and, on the other hand, on the components constituting said layer.
According to a first known embodiment, this resistive layer in the form of a cylinder, is obtained from sheet metal forming portions of a cylinder, generally two or four, interconnected along generatrices of the cylinder by splints. These latter constitute connection regions in which the portions can be superposed and are connected by means of suitable connectors, such as by cementing or riveting, which are perfectly sealed with a quantity of open surface equal to zero. To obtain a structure having suitable mechanical characteristics, the splints have relatively large surfaces.
It is known that these splints influence in an important way the acoustical characteristics of the resistive layer, because of the inhomogeneity of the overall surface, particularly in line with connection regions. Thus, these zones are inactive, and do not let sound waves pass in a manner partially to transform the acoustic energy of the sound wave into heat.
The patent FR-2.698.910 mentions this phenomenon and proposes to reduce the number of splints, preferably to omit them. It also describes a way of making the resistive layer from a single metal sheet whose side edges are interconnected by a splint so as to form a cylinder.
Even if this embodiment permits reducing the surface of the inactive zones, it does not give complete satisfaction because there still remains a splint region.
To overcome this drawback, the applicant proposes in French patent application FR-2.767.411, a process for producing a resistive layer that has no splint, in which the resistive layer comprises a cloth structurally reinforced by filament winding. However, the practice of this process is relatively complicated to obtain satisfactory mechanical characteristics. Thus, the use of resins which form the mechanical connections between the different constituents, requires the monitoring of numerous parameters, particularly temperature and pressure, which is difficult to control in situ in a repeatable manner so as to obtain the same quality of cementing over time.
Another solution proposed by the applicant in French patent application FR-01.03227, consists in emplacing the acoustically resistive layer by winding from a porous material in the form of a strip.
This manner of proceeding not only permits avoiding inhomogeneous zones from an acoustic standpoint of the porous layer, as indicated above, in the usual technique of forming two half panels, but also eliminates the necessity of splinting, the winding of the porous layer being adapted to be used for placing other layers, namely the structural layer, the porous central core, the rear reflector, so as to produce a complete acoustic panel in one piece without splints.
The absence of splints permits increasing the effective acoustic surface of the panel, decreasing its mass and reducing the time and cost of production.
If such a panel produces acoustic shock absorption of good quality, this shock absorption is however not optimum. Thus, the physical characteristics of the resistive layer are homogeneous over all the extent of the panel and are predetermined as a function of the mean value of noise which it is desired to attenuate.
However, the different modes of propagation of sound waves change in the course of their propagation in the fan channel. Thus, certain modes are subject to strong attenuation whilst others, on the contrary, are practically unaffected by the acoustic panel. Moreover, the characteristics of the noise to be attenuated are different from one place to another in the channel. As a result, an acoustic attenuation panel of the type described above, while attenuating a limited number of modes, does not permit optimum absorption of noise.
Moreover, according to this embodiment, the edges of the wound strip are substantially perpendicular to the flow in the conduit so that the strip easily peels at its edge in contact with the aerodynamic flow. Thus, not only are the acoustic qualities of the panel degraded, but furthermore the panel itself degrades and must be changed, which implies cost of maintenance and down time of the aircraft.
All the solutions of the prior art have the common object of omitting splints or regions of connection so as to increase the effect of acoustic surface of the resistive layer.
The present invention seeks to overcome the drawbacks of the prior art by providing an acoustically resistive layer for an acoustic attenuation panel having good mechanical characteristics and producing high quality acoustical damping.
To this end, the invention has for its object an acoustically resistive layer for an acoustic attenuation panel forming a conduit through which an aerodynamic flow passes, adapted for an aircraft jet engine, said panel comprising at least one acoustically resistive layer, at least one cellular structure and a reflector disposed on the side opposite the incident wave, characterized in that said acoustically resistive layer is constituted by strips disposed along the direction of said flow, interconnected by a plurality of splints ensuring strain relief, said splints having small surfaces relative to the surface of the strips so as to ensure the continuity of the homogeneous character of the quantity of open surface of the acoustically resistive layer thus formed.
By splint is meant a region of connection, inactive acoustically, between two adjacent strips which can be of any shape, for example by overlapping said strips or by using a junction strip, and of all natures, for example by cementing, riveting or the like.
According to a characteristic of the invention, the splints should have a width l at most equal to the smallest of the following values:
15 mm;
28% of the width of the strips.
In contrast to the prior art, which seeks to omit splints or connection means between the panels forming the acoustically resistive layer, the present invention seeks to multiply the number of splints. By multiplying their number, there is distributed over the largest number of regions, the absorption of mechanical force, particularly radially, so that contrary to the splints of the prior art which have relatively large surfaces for absorbing force, which necessarily alters the homogeneous character of the acoustically resistive layer, the connection zones according to the invention are very small surfaces which do not alter the homogeneous character of the acoustically resistive layer.
The small surface of each splint has the result of rendering said splints transparent to sound, which is to say that they substantially do not give rise to any disturbance in the processing of the sound waves to be attenuated, contrary to the splints of the prior art, whose large surfaces constitute inactive surfaces disturbing the processing of said waves.
According to this arrangement, the edges of the strips are disposed in the direction of the aerodynamic flow, so that they do not offer any impediment to said flow thereby limiting the risk of delamination.
According to a first embodiment, the splints are obtained by overlapping adjacent strips.
According to a second embodiment, the strips are juxtaposed and connected two by two by junction strips whose lateral edges are connected to the strips.